A peak of a communication signal represents the greatest instantaneous amplitude, magnitude, or power level exhibited by the communication signal within some period of time. The average of a communication signal represents the average amplitude, magnitude, or power level of the communication signal over that same period. The peak is greater than the average, and the ratio of the peak power to the average power (PAPR) is a parameter of interest to communication system designers.
A communication system should achieve certain goals, such as providing for communication over a certain minimum distance, providing a certain minimum data rate, accommodating a certain minimum number of users, achieving a certain minimum quality or robustness in the communicated data, consuming less than a certain maximum amount of power, and the like.
As PAPR increases, meeting these goals for the communication system's transmitting units becomes increasingly difficult. A transmitting unit's power amplifier is desirably as linear as possible, but linearity is achieved only so long as the amplitude of a communication signal remains beneath some maximum level. If the communication signal's instantaneous power exceeds this maximum level, non-linear amplification results, causing the spectrum of the communication signal to grow and exceed regulatory limitations imposed on the transmitting unit. Accordingly, the communication signal's instantaneous power should be kept below this maximum level. If a transmitting unit in this communication system is configured to accommodate its goals with a communication signal exhibiting a certain average power level, then economic and power efficiency constraints may proscribe the use of an RF power amplifier that can also accommodate an instantaneous power level significantly greater (e.g., >7 dB) than this average power level.
Different communication signal waveforms exhibit different PAPR characteristics. For example, waveforms configured in accordance with orthogonal frequency-division multiplexing (OFDM) techniques or code division multiplexing (CDM) techniques, tend to exhibit rarely occurring peaks of high power (e.g., >9 dB) relative to the average power. Little signal energy is contained in the rarely occurring high power peaks due to their scarcity. Consequently, any of a variety of conventional peak-reduction, crest-reduction, or PAPR-reduction techniques known to those skilled in the art may be employed to reduce the peaks prior to amplification. Coupled with such techniques, the amplitude of the communication signal waveform is then scaled to the point where the peak-reduced waveform's peak matches the maximum peak that the power amplifier can linearly amplify. In other words, the average power is increased by the amount of the PAPR reduction to improve the ability of the communication system to accomplish its goals.
While a variety of different conventional peak-reduction techniques are known, all intentionally distort what would otherwise be an undistorted communication signal to produce a distorted, peak-reduced communication signal for amplification in an RF power amplifier. The distortion added by a peak-reduction technique amounts to noise for the purposes of successfully communicating information between a transmitter and receiver. This peak-reduction noise may appear in the assigned frequency spectrum for the transmission unit and/or outside of this frequency spectrum. The different peak-reduction techniques are judged in accordance with their effectiveness in reducing PAPR while accommodating important constraints. These constraints include maintaining transmission unit compliance with a spectral mask which addresses out-of-band emissions and is imposed by governmental regulations, and compliance with an in-band noise constraint. The in-band noise constraint may be specified by a communication standard to which the communication system adheres or otherwise configured to indicate a noise level in the transmitted signal above which successful reception of communicated data is likely to suffer. The better peak-reduction techniques achieve greater amounts of PAPR reduction while respecting these noise constraints.
For communication signal waveforms configured in accordance with OFDM and CDM modulation techniques, including extending these modulation techniques to apply to multi-user channel access methods, such as orthogonal frequency-division multiple access (OFDMA) and code division multiple access (CDMA), significant PAPR reductions may be achieved while distorting the communication signal only a little bit. But PAPR reduction has a nonlinear relationship with respect to the amount of peak-reduction noise introduced into the communication signal. Each additional tenth of a decibel (dB) reduction in PAPR is achieved by distorting the communication signal marginally more than was required to achieve the last tenth of a dB reduction in PAPR. Where a final step in PAPR reduction process occurs to cause PAPR to reach its minimum achievable value, the final step has introduced a large amount of peak-reduction noise into the communication signal to achieve only a small amount of PAPR reduction. To achieve large reductions in PAPR, large amounts of peak-reduction noise are generated, and this peak-reduction noise should not cause the waveform to violate in-band or out-of-band constraints.
Single carrier frequency division multiple access (SC-FDMA) has become a widely used communication signal waveform, and is currently specified for Long Term Evolution (LTE) compliant communication systems. SC-FDMA may be considered to be an extension of OFDMA, where the extension consists primarily in the inclusion of a time-to-frequency domain transformation of the payload data in the transmitting unit, coupled with a complementary frequency-to-time domain transformation in the receiving unit. Compared to OFDMA waveforms, this additional transformation tends to spread the waveform's energy more evenly over time so that it exhibits a lower PAPR for amplification. In fact this reduction in PAPR compared to OFDMA or CDMA waveforms is often noted as being the primary reason SC-FDMA has become widely used in recent years. While SC-FDMA, without PAPR reduction, does not appear to achieve as low of a PAPR as can be achieved with OFDMA when paired with one of the better PAPR reduction techniques, it nevertheless gets close enough that the inclusion of PAPR reduction circuits has usually been deemed not worth the cost.
Unfortunately, communication systems are beginning to adhere to newer communication standards that have the effect of changing the character of SC-FDMA waveforms that complied with older standards. As communication systems push from LTE Rev 008 to Rev 009 and beyond (i.e., LTE-A), the compliant communication signal waveform will exhibit an increased PAPR from that observed in LTE Rev 008. This increased PAPR is attributable at least in part to one or more of an expanded bandwidth, aggregated inter-band and intra-band component carriers, and expanded multiple-input and multiple-output (MIMO) transmission modes. Accordingly, such newer-version SC-FDMA waveforms will fail to exhibit the acceptably low PAPRs that are compatible with power-efficient and economically beneficial RF power amplifiers, such as those exhibited by older-version SC-FDMA waveforms.
What is needed is an improvement in communication signal peak management that will be applicable to a variety of FDMA and other waveforms, including more modern, expanded-bandwidth SC-FDMA waveforms, and that will conform with out-of-band and in-band noise constraints, yet permit the introduction of increased levels of distortion in the undistorted waveform so that communication signal PAPR may be reduced to the low level that is compatible with power-efficient and economically beneficial RF power amplifiers.